In 1991, Moraes and colleagues from Columbia University reported a new form of mitochondrial DNA (mtDNA) abnormality-mtDNA depletion- in 4 children. In contrast to the previously reported qualitative defects, depletion, as the name implied, is a quantitative abnormality. Although greatly reduced in number, the mitochondrial genome does not appear to harbor any qualitative defects; therefore, mtDNA depletion is considered the primary etiologic mechanism. In affected children, the syndrome of mtDNA depletion is clinically heterogeneous and fatal. Depletion of mtDNA may occur in specific tissues or in several organs, including muscle, brain, liver, and kidney. Affected tissue show a paucity of mtDNA -encoded translation products, multiple respiratory chain defects, and deficiency of cytochrome c oxidase (COX) activity on histochemical studies. The disease is not uncommon, and to date, we have identified and studies 50 children with mtDNA depletion. The proposed project aims to further characterized and to identify the genetic defect(s) in mtDNA depletion syndrome and two other disorders: Navajo neuropathy and Alpers syndrome. Seemingly an eclectic mix, these disorders actually share several common features: they affect the liver and nervous system of children, they are autosomal recessively inherited, and most importantly, all are associated with depletion of mtDNA. We hypothesize that the lack of mtDNA plays a pathogenetic role in these hepatoneural syndromes. Using a combination of approaches, including morphologic, biochemical, molecular, and linkage analyses, we will test several hypotheses. Data gathered from this study will broaden our knowledge of the role of mtDNA depletion in human disease and will resolve diagnostic issues, especially in patients with hepatopathy and mtDNA depletion and those with Alpers' syndrome. Discovering the genetic basis of mtDNA depletion is important clinically, for it allows earlier diagnosis and more accurate genetic counseling. Knowing the genetic defect will also facilitate the design of therapies for what are now fatal disorders. Scientifically, by elucidating the pathogenetic mechanism underlying mtDNA depletion, we will gain insights into the interrelationship between nuclear and mitochondrial genomes.